Sizing an Air Filter for Central Air Conditioning

The air filter must be large enough not to restrict air flow when loaded with dirt and particulates.  Undersized filter, grill and return air duct systems are the most common air conditioning installation problems.

Your filter, grill and duct should never be undersized.  Most residential systems need 400 CFM (cubic feet per minute) of air per ton of cooling.  This air flow can be measured, but most service companies do not have the instrumentation.  Rule of thumb is 144 square inches of open area per ton.  A five ton system would need a minimum of 720 square inches open area, but seldom have larger than 25 x 25 (625 square inches).  Adding a high efficiency filter to an undersized return air aggravates an already bad situation.

If you suspect a problem, find a contractor with an air flow hood or hot wire anemometer to measure the air flow